Seismic-refraction and seismic-reflection profiles reported elsewhere indicate that the Agulhas Plateau is a continental fragment that was rifted from the Falkland Plateau and intruded by basaltic magmas during the early opening of the South Atlantic. Dredge hauls recovering a characteristic suite of low- to high-grade continental metamorphic rocks, indurated sedimentary rocks, and basaltic igneous samples support this origin and evolutionary history. Both the petrography of the gneiss specimens and K-Ar ages of 1,074 ± 36 m.y. and 478 ± 17 m.y. on fresh biotites from two of the rocks are compatible with petrography and ages of the formerly adjacent continental rocks of southern Africa, the Falkland Islands, Falkland Plateau, and Antarctica.